Sorry to be blunt but....You should never let your dogs roam free, no matter how safe you think it is...what has happened just goes to prove why.
Dogs are never 100% safe or trustworthy, they are animals.
And as for the RSPCA, i would not be surprised if they decided to prosecute you for allowing the dog to roam free, causing suffering to the animal by doing so, the RSPCA are even less trustworthy than animals